1. Field of the Invention
The present invention relates generally to methods of controlling transmit power for a user call migrating between entities in a base station of a wireless communication systems or networks.
2. Description of the Related Art
Wireless communication systems typically include a plurality of base stations, where one or more base stations may serve a geographic area commonly referred to as a cell. Users of the wireless communication system may communicate with each other and/or with the communication system. The physical equipment at the base stations may be referred to as system equipment. In wireless communication systems, communication can occur between two mobile stations (e.g., two cell phones, wireless computers, PDAs, etc.) and/or between mobiles and system equipment.
The communication system typically may operate in accordance with established rules promulgated by governmental and/or industry groups. These rules are typically established as standards by which wireless communication systems are to be in compliance with. The standards may contain various protocols that dictate the operation of the wireless communication systems. Generally, a protocol may be understood as a set of rules that may specify how communication (voice and/or data) within a given communication system may be initiated, maintained and/or terminated.
A mobile located in a particular cell being served by a base station communicates with other mobiles and/or with other communication systems via the base station. The signals from the mobile may be transmitted to a receiving mobile or to another communication system via the base station of the cell within which the mobile is located.
As a mobile move through cells being served by different base stations, communication from the mobile may be handled by different base stations. For example, communication for a mobile moving from one cell to another cell may be transferred from one base station to another base station of the cell. The process in which communication for a mobile is transferred from one base station (i.e., the ‘original’ base station) to another base station (i.e., the ‘target’ base station) is known as “handoff.” A mobile located at the edge of a serving cell and in the vicinity or near a target cell typically will request a handoff to the target cell, depending on the relative strength of communication signals of neighboring base stations.
In wireless communication systems or networks, the base stations (also commonly referred to as a Node-B) typically may transmit pilot signals periodically. The mobiles receive these pilot signals and may be configured so as to measure the strength of these pilot signals. A mobile may request a handoff to one or more base stations based on the relative measured strengths of the pilot signals of the base stations. The network may also trigger a handoff procedure based on periodic measurement reports received from a given mobile, or for other reasons such as congestion in a cell that a given mobile is connected to.
One type of handoff used in several spread spectrum-based wireless communication systems such as Code Division Multiple Access (CDMA) systems is referred to as a soft handoff. In a soft handoff, communication with the target base station may be established prior to communication with the original base station is terminated. Also during soft handoff, the mobile may typically be in simultaneous communication with greater than one base station. In this manner, no interruption of communication to and from the mobile occurs. Eventually, when the soft handoff has terminated, the mobile communicates with another mobile or another communication system via the target base station.
Power control is used in both the downlink (base station to mobile) and uplink (mobile to base station) of CDMA systems to reduce interference and/or to ensure a desired target block error rate. For each dedicated channel, there is a closed loop to control transmitter power. This is commonly known as inner loop power control.
For downlink power control, the mobile measures received signal to interference ratio (SIR) and may issue an up power command in the uplink if the measured SIR is below a given threshold. Otherwise, the mobile issues a down power command in the uplink to the base station where measured SIR exceeds the threshold. This power control command (TPC) may be transmitted periodically to the base station(s) to which mobile is connected. The period of TPC may be referred to as a slot or power control group. If a mobile is in soft handoff, all base stations in communication with the mobile receive the TPC command that is transmitted in the uplink.
For uplink power control, each base station measures received SIR of a given mobile and may issue an up power command on the downlink to the mobile, if the measured SIR is below a given threshold. This threshold may be referred to as ‘SIR target’. If the measured SIR exceeds SIR target, the base station issues a down power command. The SIR target itself may be adjusted based on the number of errors in the received blocks from the mobile. If a received block is erroneous the SIR target is raised, otherwise the SIR target is lowered where the received block is not erroneous.
During soft handoff, each base station in communication with the mobile does not decode the same TPC command transmitted in the uplink (UL) due to the difference of the wireless channel paths between the mobile and each base station. If the channel is not fading between the base station and the mobile, the probability of errors on the decoded TPC commands by the base station is relatively low, and is relatively high in case of deep fading. In the case of deep fading, the TPC commands received are generally random, thus disrupting inner loop power control and potentially unnecessarily increasing the transmitted power of the weak link. Because the inner loop power control processing at each base station responds to the TPC commands it receives, cumulative TPC bit errors may cause the base station transmit power to drift substantially from the target power, potentially leading to capacity loss and/or call drops during handoff. Since desired downlink capacity may be achieved where the transmit power values of all base stations in the mobile's active set are the same or nearly the same, a mechanism to synchronize the transmitted power levels of each base station in the active set is desired.
A power adjustment procedure known as power balancing is used in most if not all wireless standards to remove what is known as the random walk effect. The random walk effect refers to the downlink transmit power level drifting away from the desired power level due to recovered power control command errors in the uplink receiver. This is especially true for the weak links in soft handoff scenarios. For weak links, the power control command error rate is high. Therefore when the downlink transmit power level follows the erroneous power control command received from the uplink, it starts to wander away from the desired level. For high error rate of power control command, the error pattern looks random, therefore the trajectory of the downlink power level resembles that of the foot steps of a drunken man, hence the name ‘random walk’. Since the drifting can be detected by comparing the actual long-term transmit power level with the desired one, power balancing procedure is started once this condition is detected, by pulling the downlink power levels in the direction such that its long-term average goes back to the desired power level.
There are several situations within a base station where a user call being served by that base station needs to be moved from one physical entity (i.e., a board) in the base station to another board. The reasons for moving a call from one entity to another entity in the Node-B may include, but are not limited to, reconfiguration of the data size to be transmitted to the mobile and certain handoff scenarios. In the data size reconfiguration scenario, the physical entity in the Node B currently handling the user call cannot deal with the new data size; thus the call needs to be transferred to another physical entity at the Node B capable of delivering the data. In an example handoff scenario, a call may be moved from one entity to another entity in the Node B where the mobile needs, after moving to a new sector, to be physically located on an entity that deals with the new sector.
The procedure of moving a user call between entities in a Node-B may be referred to as “call migration”. It is desirable that call migration be performed in a way that is substantially transparent to any entity outside of the base station and in a substantially short period of time (i.e., it should be at least an order of magnitude smaller than the reconfiguration time for migrating the call from one entity to another in the Node-B). In other words, the data to be received and transmitted, along with the power to be transmitted, should be able to continue in a continuous manner with no interruption.
Additionally, performing call migration transparently means that the power of the last slot to be transmitted from the ‘old’ physical entity (previously handling the user call) is to be an input to the computation of the power for the first slot to be transmitted from the new physical entity taking the migrated call. Since communication latency between physical entities in a Node-B typically may be about one to two orders of magnitude greater than slot duration reducing and/or potentially eliminating this latency for call migration between entities may be desirable.